Adding a basic fix and a resulting spec

lib/gitlab/diff/suggestion_diff.rb

@@ -18,13 +18,17 @@ module Gitlab
       private
 
       def raw_diff
-        "#{diff_header}\n#{from_content_as_diff}#{to_content_as_diff}"
+        "#{diff_header}\n#{from_content_as_diff}#{spacer}#{to_content_as_diff}"
       end
 
       def diff_header
         "@@ -#{from_line} +#{from_line}"
       end
 
+      def spacer
+        "\n" unless from_content_as_diff[-1, 3] == "\n"
+      end
+
       def from_content_as_diff
         from_content.lines.map { |line| line.prepend('-') }.join
       end

spec/lib/gitlab/diff/suggestion_diff_spec.rb

@@ -51,5 +51,22 @@ describe Gitlab::Diff::SuggestionDiff do
         expect(diff_lines[index].to_hash).to include(expected_line)
       end
     end
+
+    describe 'when the suggestion is for the last line of a file' do
+      it 'returns a correct value if there is no newline at the end of the file' do
+        from_content = "One line test"
+        to_content = "Successful test!"
+        suggestion = instance_double(Suggestion, from_line: 1,
+                                                 from_content: from_content,
+                                                 to_content: to_content)
+
+        diff_lines = described_class.new(suggestion).diff_lines
+        bad_string = "-One line test+Successful test!"
+
+        diff_lines.each do |diff_line|
+          expect(diff_line.to_hash).not_to include(bad_string)
+        end
+      end
+    end
   end
 end